Purifying and washing process



UNITED STATES PATENT OFFICE.

SAMUEL E. SHEPPARD, OF ROCHESTER, NEW YORK, ASSIGNOR TO EASTMAN KODAK COMPANY, OF ROCHESTER, NEW YORK, A CORPORATION OF NEW YORK.

PURIFYING AND WASHING PROCESS.

No Drawing.

To all whom it may concern Be it known that I, SAMUEL E. SHEPPARD, a subject of the King of GreatBritain, residing at Rochester, in the county of Monroe and State of New York, have invented certain new and useful Improvements in Purifying and lVashing Processes, of which the following is a full, clear, and exact specification. I

This invention relates to the washing and purifying of cellulose esters and it finds its most important application in the treatment of cellulose nitrate to remove the unfixed acid and unstable acid compounds which are associated therewith.

The objects of my invention are to effect this purification and washing more thoroughly, in shorter time, using less water, and less space than in prior processes; to utilize cheap ingredients which can be used repeatedly; to employ a solution of a salt derived from a strong base and a strong acid whenever the process requires a non-hydrolyzable salt; to employ a saline leach of sufficient strength to osmotically compress the treated materialand extract water, acid, and unstable acid compounds therefrom while depressing the dissociation of these acid substances as well as the stable esters; to utilize soft'water for washing as well as hard water; to provide a process which will be effective at ordinary .or room temperatures yet may be usefully combined with heating operations; to obtain these advantages without decomposing the relatively stable esters which are undergoing treatment; and to obtain a product which shall be, when necessary, practically, free from particles of insoluble opaque mineral matter. Other objects will hereinafter appear.

Since, my invention at present finds its widest application in the purification of nitrated cellulose fiber I shall describe its ap plication to that material, by way of illus- .tration. As is Well known, the cellulose, usually cotton, is nitrated in a bath of concentrated nitric and sulfuric acids. nitration has progressed to the desired'degree, the mixture is subjected to a centrifugalizing action to wring quigas much acid as possible. It is then plunged, drowned or soused in a bath of water to remove some of the acid, keep the mass cool, and prepare it for the subsequent purification operations. The unpurified nitrated cellulose at this Specification of Letters Patent.

Application filed June 5, 1918.

1 their own object by causing a decomposition When Patented Nov. 2, 1920.

Serial no. 238,400.

stage of the process consists of fibers or fibrous fragments composed of the higher and relatively stable cellulose nitrates together with adherent or mechanically held acid and capillarily retained or adsorbed acid. There may be present also unstable so-called lower cellulose nitrates together with some cellulose sulfate or cellulose-sulfuric-acid and it is probable these contain part of the adsorbed acid. In order to make the final product stable and for other reasons, it has been long desired to eliminate all other of these substances except the higher relatively stable cellulose nitrates. It has been attempted to remove these impurities by prolonged washing in successive changes of water or successive boilingsin different changes of water, either of these methods being at times supplemented'by pulping and treatment with a dilute alkaline bath to neu- 5 trali'ze any acid that maystill remain. S'uch processes, although widely used, have been open to several objections. The amount of water used has been very great; the time of the operations has been prolonged, thus keepinga large mass of material under various stages of treatment in the factory and requiring a relatively large space and equipment; and my experiments show that ,the elimination of the impurities has not always been as complete as was believed. Furthermore, hard water or water containing mineral substances which give it a mild alkaline reaction was generally thought to be necessary for washing because of its possible neutralizing properties. The known methods have the additional disadvantage that the prolonged washing, boiling or neutralizing operations tend to partially defeat of, the relatively stable cellulose nitrates which are being purified.

I have discovered that if the substances to be purified are treated with a strong leach of a nearly neutral saline solution, preferably nearly'saturated, and then washed, there will be removed from 50 to per cent. more unfixed acid than is removed by plain washing in soft water. Also the pur1fication can be effected in to 9; of the time whatever the nature of the Water. Moreover, the number of changes of water is greatly reduced; so that less water, space and apparatus are required for a given output and soft Water may. be used as well as be combined with the known boiling methods with a substantial reduction of the washing periods between boilings. Furthermore the saline leach can be used as a semi-stabilizing stop bath so that boiling may be taken up at any desired time after nitrating.

In one form of myinvention, which I cite by wayof a specific illustration, the soused nitrated cotton is wrung in a centrifugal machine for twenty minutes while being sprayed with a nearly saturated and nearly neutral solution of sodium sulfate. It is then removed to a bath of similar strong sodium sulfate solution and leached for one hour. Next it is wrung and drained between squeeze rolls or in a centrifugal for fifteen minutes. These operations of leaching in successive baths and wringing and draining .off the solutlon between baths are then repeated; so that in a typical case the cotton will receive a total of four or five sulfate leachings of one hour each. During any of these leaching operations the baths may be warmed to accelerate the action. a con venient temperature being 35 C. However, room temperature is generally adequate. Furthermore, I may sometimes (but not necessarily) .make one of the leaching baths, preferably one of the intermediate baths, mildly alkaline by adding small quantities of sodium carbonate. This will reach and neutralize acid that may be contained in the interior of balls or clumps of the cotton. The subsequent wringing and leaching steps remove any remaining carbonates along with the other substances which are leached out. The purified nitrated cotton is finally washed, for instance by simultaneously wringing in a centrifugal machine and spraying with water for fifteen or twenty minutes, the sulfate being thoroughly removed. The whole process from sousing to final washing does not require more than half a day as compared with one or two weeks treatment in the usual washing processes other than boiling methods and is, therefore, adapted to large scale work.

Wringing operations between leachings are sometimes inconvenient, requiring as I they do centrifugals, rolls or other maoughly washed. In this form of my invention the action may also be hastened, if desired, by warming the leaching baths to about 35 C., the precise temperature being unimportant. l/Vhen, as in this case, no wringing is used between leachings, it is preferable to make one of the baths, say the sixth, mildly alkaline with small amounts of sodium carbonate. This process does not take over twelve or fifteen hours, particularly if a centrifugal and water spray be available for the final removal of the sulfate from the cotton. With hard water of permissible alkalinity the same total acid may be removed, but it requires much more water, many more changes, and therefore greatly prolonged treatment as compared with my method.

As regards the particular neutral leaching solution, I prefer a nearly saturated solution of sodium sulfate because it has the necessary properties and is at the same time a cheap by-product from the manufacture of nitric acid. If the commercial sodium sulfate contains free acid, it is easy and inexpensive to neutralize it.

Moreover, the used leaching solution may be treated to neutralize the acid which it has extracted and then can be employed again. I may add, for example, comminuted calcium carbonate and filter, thereby regenerating the bath. I find that this neutralizes the acid substances without, however, making the bath too alkaline. It becomes practically neutral, in fact, its alkalinity being very small and comparable to that of hard water. It is especially noted that this regeneration requires no analysis or titrations by skilled persons but unskilled labor may be employed. This regeneration of each bath may be repeated indefinitely.

I prefer a salt, like sodium sulfate, derived from the union of a strong base with a strong acid because such a salt gives a 110 substantially neutral solution and does not hydrolyze to make the solution alkaline or acid, as is the case when a weak acid is combined with a strong base or a weak base combined with a strong acid. The initial neu- 115 trality of the leaching solution reduces any tendency to decomposition of the relatively stable esters.

The leaching step is-ordinarily carried out at normal or room temperatures and 2 pressures. In some cases, however, as above indicated, I may heat the leaching bath to accelerate the action or I may heat the water by air under pressure, or such agitation may take place just prior to or subsequent to a leaching operation. From the physical-chemistry standpoint it appears that the remarkable action which takes place in the leaching steps may be ex lained as follows:

he acid nitrocellulose consists of fibers or fiber fragments containing inner filamentous canals, the aggregate of fibers having a ver large surface in proportion to its mass, he material of the fibers consists of the stable cellulose esters and some less altered cellulose carrying both adherent or mechanically held acid and adsorbed acid. The adsorbed acid is probably loosely combined in an ionizable and readily hydrolyzable complex with hydrated cellulose residues which exist as a membrane at thesurface of the fibers. All

' these substances tend to a definite equi-- librium in respect of acid residue undergiven conditions of temperature, concentration, etc., and this equilibrium is quite different in water than when my saline leach is employed. The adherent acid and the adsorbed acid will come to equilibrium relatively rapidly, whereas adjustment is slow between the adsorbed acid and, the stable esters. In the prolonged boiling and neutralizing steps of the prior art the equilibrium between the stable esters and the unfixed acids is liable to be altered in such a direction that considerable quantities of.

the stable esters tend to decompose.

In my process, however, the removal of the adherent and adsorbed acids is efi'ected osmotieally without injuriously affecting the equilibrium between the stable esters and the unfixed acids, this efl'ect being due in part to the isohydric depression of the dissociation (of ionization and swelling) of the adherent acid and of the adsorbed acid substances by the mass actiorrof the concentrated, neutral salt. The osmotic action produces a powerful contraction of the osmotic cells and membranes constituted by the fibers and fiber fragments, which contraction is an aid in removing any acid held by the fiber. This also tends to check decomposition of the true cellulose esters.

Of course a correct knowledge I of the physico-chemical theory of my process is not indispensable to its practical application, but it gives an insight into how I obtain the new results which differ from the prior art, particularly those processes-which employ so-called stabilizers.

The action of my leaching bath is independent of the degree to which the nitration,has, been carried out, if said degree be within the limits of commercial practice.

The presence of unaltered cellulose is also immaterial in the purification steps.

, to the removal of the unfixed acid and acid Nitrocellulose treated according to my method is more quiet in'the burning test than theaverage nitrocellulose obtained by prolonged Washing alone. It dries without trouble at the usual temperature; is soluble in the acetone-methyl alcohol mixture and gives a viscosity ,at least equal to and generally better than that of similar products in the prior art. The. product is freer from opaque mineral matters which are sometimes introduced into the film by prolonged washing with hard waters. This freedom from opaque particles is particularly important where the nitrocellulose is to be worked up into transparent films for photographic purposes.

While it is generally preferred to wash out the (residue of) leaching salt remaining after the free acid is removed, yet in some instances even this additional operation is unnecessaryor may be much abbreviated, because the presence of the salt is harmless in many of the compositions into which nitrocellulose enters and adds to the stability.

' l have described my invention as applied to the removal of the acid impurities from nitrocellulose by way of illustration but my research has shown that it is also applicable substances which remain after esterefication when making analo ous material under similar conditions. also note that my process can ,be used equally well whether I the original cellulose which was nitrated was derived from cotton, wood, paper or any usual or referred source.

While a p urality ofleachings are generally advisable, one such leaching in strong salt solution Wlll greatly shorten thetime of subsequent washing and removal of acid. I may also alternate leaching operations with washing steps instead of mere draining. steps in order to accentuate the alternate osmotic compression and hydration.

Having-thus described my invention what I claim and desire to secure by Letters Patentof the United States is:

l. Theprocess of removing unfixed acid and acid substances from bodies of cellulose esters having large surface in proportion to their mass, whlch includes the steps of treating the same with a strong solution of a salt and then washing out. said salt therefrom while said bodies remain in a substantially undissolved state.

2. The process of removing unfixed acid and acid substances from bodies of cellulose esters having large surface in proportion to ing the same witha strong, nearly neutral solution of a salt and washing the same while said bodies remain in a substantially undissolved fstate. p 3. The process of removing unfixed acid and acid substances from bodies of cellulose esters having large surface in proportion to their mass, which includes the step of treating the same with a strong solution of sodium sulfate.

4. The process of removing unfixed acid and acid substances from bodies of cellulose esters having large surface in proportion to their mass, which includes the step of repeatedly treating the same with a strong which includes the step of treating the same with a strong solution of a salt formed from a strong base and a strong acid and finally washing the same while said nitrated cellulose remains in a substantially undissolved state.

7. The process of removing unfixed acid and acid substances from nitrated cellulose,

which includes the step of treating the same with a strong solution of sodium sulfate.

8. The process of removing unfixed acid and acid substances from nitrated cellulose,

which includes the steps of treating the same ,with a strong, nearly neutral-solution of sodium sulfate and washing the same.

9. The process of removing unfi'ired acid and acid substances from nitrated cellulose, which includes the steps of treating the same in a nearly saturated solution of sodium sulfate and washing the same with water.

10. The process of removing unfixed acid and acid substances from bodies of cellulose esters having large surface in proportion to their mass, which includes the steps of alternately treating the same with strong salt solution and draining such solution therefrom.

11. The process of removing unfixed acid and acid substances from bodies of'cellulose esters having large surface in proportion to their mass, which includes the steps of treating the same with a strong solution of a salt, removing a large part of such-solution from said bodies by subjecting them to pressure and finally washing the same while said bodies remain in a substantially undissolved state.

12. The process of removing unfiXed acidv and acid substances from nitrated cellulose, which comprises treating the same repeatedly with a strong solution of sodium sulfate.

13. The process of removing unfiXed acid and acid substances from nitrated cellulose,

which comprises soaking the same in a strong solutlon of sodlum sulfate and drain- 1ng such solution therefrom.

14:. The process of removing unfixed acid and acid substances from nitrated cellulose, which comprises repeatedly subjecting the same to treatment with strong sodium sulfate solution and removing a large part of such solution between each treatment and finally washing the same.

15. The process of removing unfixed acid and acid substances from nitrated cellulose, which comprises the steps of successively treating the same to the action of strong sodium sulfate solution, during the intervals between said successive treatments, removing a large part of said solution by pressure, and finally washing out said solution while keeping said nitrocellulose under pressure.

16. The process of removing unfixed acid and acid substances from nitrated cellulose, which includes the step of treating the same with warm strong sodium sulfate solution.

17.' The process of removing unfixed acid and acid substances from nitrated cellulose, which includes the steps of treating the same successively to a plurality of baths of strong salt solution, one of said baths being mildly alkaline, and others substantially neutral.

18. The process of removing unfixed acid and acid substances from bodies of cellulose esters having large surface in proportion to the-mass, which comprises repeatedly treating the same with strong salt solution, and re eatedly neutralizing the acid and acid su stances taken up during said treatments by said solution.

19. The process of removing unfiXed acid and acid substances from nitrated cellulose, which comprises treating successive batches of said nitrated cellulose with a strong sodium sulfate solution, and neutralizing the acids and acid substances taken up by the solution during said treatments by adding calcium carbonate to said solution and filtering the same between said treatments.

20. The process of removing unfixed acid and acid substances from bodies of cellulose esters having large surface in proportion to their mass, which includes soaking the same in a strong solution of a salt formed from a strong base and a strong acid and finally washing out said salt therefrom whilesaid bodies remain in a substantially undissolved state.

21. The process of removing adherent and adsorbed acid and acid substances from organic compounds which have large surface in proportion to mass, said surface comprising osmotic .membranes, which includes the steps of leaching said compound in a saline bath sufficiently concentrated to osmotically compress the material and extract water and unfi red acids while isohydrically depressing the dissociation of the compounds, and

finally washing the same while said compounds remain in a substantially undissolved state.

22. The process of removing unfixed acid and acid substances from bodies of cellulose esters having large surface in proportion to their mass, which includes the steps of treating the same in a strong salt solution, and wringing and washing the same while said bodies remain in a substantially undissolved state.

23. The process of treating soused nitrated cellulose, which includes the steps of simultaneously wringing said nitrated cellulose and washing the same with a strong salt solution. I

24. The process which comprises sousing nitrated cotton, wringing said nitrated cotton and washing it with a strong salt solution, leaching said nitrated cotton in a bath of strong salt solution, wringing and draining said solution from said nitrated cotton, repeating said leaching, wringing and drainingoperations, and finally washing and wringing said nitrated cotton.

25. The process which comprises, sousing nitrated cotton, wringing said nitrated cot- .ton while spraying the same with a strong alkaline, and finally wringing and spraying said nitrated cotton with water.

Signed at Rochester, New York, this 27th day of May, 1918.

SAMUEL E. SHEPPARD. 

